1. Field of the Invention
This invention pertains generally to circuit interrupters and, more particularly, to such circuit interrupters structured to trip open separable contacts responsive to trip conditions, such as for example, arc fault, ground fault and/or overvoltage conditions. The invention also relates to methods of processor phase synchronization.
2. Background Information
Circuit interrupters include, for example, circuit breakers, contactors, motor starters, motor controllers, other load controllers and receptacles having a trip mechanism. Circuit breakers are generally mature and well known in the art. Examples of circuit breakers are disclosed in U.S. Pat. Nos. 5,260,676; and 5,293,522.
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which is heated and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system. An armature, which is attracted by the sizable magnetic forces generated by a short circuit or fault, also unlatches, or trips, the operating mechanism.
In many applications, the miniature circuit breaker also provides ground fault protection. Typically, an electronic circuit detects leakage of current to ground and generates a ground fault trip signal. This trip signal energizes a shunt trip solenoid, which unlatches the operating mechanism, typically through actuation of the thermal-magnetic trip device.
Recently, there has been considerable interest in also providing protection against arc faults. Arc faults are intermittent high impedance faults, which can be caused, for instance, by worn insulation between adjacent conductors, by exposed ends between broken conductors, and in other situations where conducting elements at different potentials are in close proximity. Because of their intermittent and high impedance nature, arc faults do not generate currents of either sufficient instantaneous magnitude or sufficient average heating or RMS current value large enough to trip the conventional circuit interrupter. Even so, the arcs can cause damage or start a fire if they occur near combustible material. It is not practical to simply lower the pick-up currents on conventional circuit breakers, as there are many typical loads that draw similar currents and would, therefore, cause nuisance trips. Consequently, separate electrical circuits have been developed for responding to arc faults. See, for example, U.S. Pat. Nos. 5,224,006; and 5,691,869.
The operation of the protective algorithm of some residential circuit breakers is designed assuming that the processor (e.g., microcontroller) captures certain pieces of critical system data coincident with particular phase angles of the line-to-neutral source voltage. If some critical piece or pieces of data are collected at improper phase angles relative to the line-to-neutral voltage, then there is a possibility that the protective algorithm may misidentify normal circuit operation as being an undesirable or hazardous condition and unnecessarily interrupt the power circuit.
UL 1998, a standard for software in programmable components, discloses measures to address microelectronic hardware failure modes, including physical failures of a microelectronic hardware clock. This standard also discloses that software addresses identified risks, that frequency monitoring denotes a fault/error control technique in which clock frequency is compared with an independent fixed frequency, and that software maintains a Risk Addressed state upon detection of a condition that is capable of resulting in a risk.
There is room for improvement in circuit interrupters.
There is also room for improvement in methods of processor phase synchronization.